1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device, such as an EEPROM (Electrically Erasable and Programmable Read Only Memory) or a flash memory, having memory cells for storing data by the amount of an electrostatic charge on a channel of a transistor. These memories can hold data when power is not supplied.
2. Description of the Related Art
Semiconductor memory devices for storing data with elements integrated on a semiconductor substrate are broadly divided into two kinds of volatile memories which can hold data only when power is supplied, and nonvolatile memories which can hold data when power is not supplied. These memories are further divided according to methods or uses. At present, of the nonvolatile memories, a flash EEPROM which can be electrically writable and erasable has been widely used. As a device for the flash EEPROM, there has been known a floating memory cell which has a floating gate formed on a channel of a MOS transistor, a periphery of which is insulated by an oxide film or the like. Such a floating memory cell changes a gate threshold voltage (hereinafter, referred to as Vt), with which a source-to-drain current of the MOS transistor starts to flow, by injecting or releasing electrons into or from the floating gate, thereby storing data.
FIG. 19 is a cross-sectional view of a stacked flash EEPROM memory cell which is widely used at present. There is disclosed a transistor structure which has a floating gate FG and a control gate CG for potential control formed on a substrate Sub, and a source S and a drain D arranged at both ends. Further, an ONO film is formed between the control gate CG and the floating gate FG, and a SiO2 film with little crystal defect formed by thermal oxidization is formed between the floating gate FG and the substrate Sub. In an actual memory array, multiple memory cells are successively arranged in vertical and horizontal directions on a semiconductor substrate, and a potential Vg, Vs or Vd is supplied to the control gates CG, the sources S or the drains D by word lines WL, source lines or bit lines.
FIG. 20 schematically shows a peripheral portion of a memory array of a conventional flash EEPROM. Further, FIG. 21 is a circuit diagram showing the inner parts of a memory array 101 or a reference cell 105 of FIG. 20. As shown in FIG. 21, the memory array 101 has a plurality of memory cells M00 to Mnm in an array shape in vertical and horizontal directions, and the control gates of the memory cells are correspondingly connected to the word lines WL0 to WLn. Each of the word lines WL0 to WLn serves as a common node for the memory cells horizontally arranged. For example, the control gates of the memory cells M00, M01, M02, . . . , and M0m are connected to the word line WL0. Further, the drains of the memory cells are correspondingly connected to the bit lines BL1 to BLm. Each of the bit lines BL1 to BLm serves as a common node for the memory cells vertically arranged. For example, the drains of the memory cells M01, M11, M21, . . . , and Mn1 are connected to the bit line BL1. Further, these cells are also connected to the source line SL0 simultaneously. The word lines, the bit lines, and the source lines can supply potentials by a row decoder 102 and a column decoder 103 shown in FIG. 20.
In FIG. 21, a reference cell R0, which generates a voltage or current to be a judgment reference when data is read out from each of the memory cells M00 to Mnm, is connected to a word line RWL and a source line RSL, like the memory cells. Further, the reference cell R0 is also connected to a sense amplifier SA through a bit line RBL. As the reference cell R0, the same nonvolatile memory (flash EEPROM) as the memory cell, not a CMOS transistor, is used. In general, the voltage or current to be the reference requires high accuracy, and thus an allowable range of a variation in characteristic for the reference cell which generates the voltage or current to be the reference is narrow. When a CMOS transistor is used for the reference cell, a manufacturing variation in characteristic inevitably occurs. This variation leads to a reduction in yield and an increase in chip area due to an advanced circuit design for increasing the allowable range of the variation in characteristic. In contrast, when the nonvolatile memory is used, it is possible to meet the manufacturing variation in characteristic of the reference cell through the Vt adjustment of the reference cell at the time of inspection. Therefore, unlike the above-described case, the reduction in yield or the increase in chip area does not occur. In addition, a variation in characteristic of a reading circuit such as a sense amplifier can be adjusted by the reference cell using a nonvolatile memory, and thus a larger operation margin or enhancement of a specification can be expected. It is desirable to reduce an inspection cost of a memory device using a nonvolatile memory, like a memory cell, for the reference cell.
For the inspection of the nonvolatile memory, in general, screening which involves high-temperature storage is executed to assure data holding characteristics. FIG. 22 is a schematic view showing an inspection flow of a conventional memory device. In this case, after the optimum value of Vt of the reference cell is set, the writing/erasure/reading inspection and the high-temperature storage are performed in sequence. At that time, since high-temperature stress is applied to the nonvolatile memory used as the reference cell, Vt is changed. In order to perform the inspection after the application of the stress, it is necessary to correct the Vt change (restore Vt to the set value at the beginning of the inspection).
FIG. 23 is a flowchart showing a method of setting and restoring initial Vt of the reference cell of the conventional memory device. A cell current is first measured, and then, the measured value of the cell current is different from a desired current value, a bias voltage is applied to the reference cell so as to change Vt of the reference cell (hereinafter, this operation is referred to as a write operation into the reference cell). The cell current measurement and the write operation are repeated until the measured value of the cell current reaches the desired cell current value. A current of the reference cell when initial Vt setting is completed is stored, and, at the time of the Vt restoration, the cell current measurement and the write operation are repeated until the measured value of the cell current is equal to the stored current value.
Moreover, JP-A-2004-39075 is an example of the related art.
However, as described above, the characteristics of the reference cell should have high accurate, and thus it is necessary to accurately restore Vt of the reference cell to Vt at the time of initial setting. For this reason, the verification by the current of the reference cell should be repeated by performing the write operation under the low bias voltage condition, and by gradually changing Vt. In general, since it takes much time for the current measurement, the repetition of the current measurement plural times causes an increase in inspection time and product cost. An object of the invention is to reduce time required for a Vt restoration to be repeatedly performed during an inspection process. Further, another object of the invention is to reduce time required for initial Vt setting of a reference cell.